1. Field of the Invention
The present invention relates to a method of preparing an oxide superconducting film, and more particularly, it relates to a method of preparing an oxide superconducting film which is formed on a base material by a vapor phase method.
2. Description of the Background Art
In order to enable application of an oxide superconductive material to a conductor, an oxide superconducting film may be formed on a base material. When such base materials are made of single-crystalline materials, for example, it is possible to obtain various elements, provided with oxide superconducting films, which are employed in the field of electronics. When a base material is made of a flexible long material, it is possible to obtain an oxide superconducting wire.
Among oxide superconductive materials, a BiSrCaCuO superconductive material is known by its high critical temperature of superconductivity.
In order to obtain a BiSrCaCuO superconductor by a solid phase method, lead or a compound of lead is added to a BiSrCaCuO superconductive material, which in turn is heat treated, thereby effectively improving the critical temperature of superconductivity.
On the other hand, in order to obtain a BiSrCaCuO superconductor by a vapor phase method, lead is added to a superconductive material. An oxide superconducting film having a high critical temperature is successfully prepared by:
(1) a method of adding lead during film formation and heat treating the material; or
(2) a method of depositing a Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 phase, which has a critical temperature of 80K, in a crystallographically oriented state, thereafter laying lead on the same and heat treating the material, thereby converting the Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 phase to a Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 phase, which has a critical temperature of 110K (1989 Heisei Gannen Dai 36 Kai Oyo Butsurigaku Kainkei Rengo Koen-Kai Koen Yoko-Shu, 4p-F-2, "BPSCCO Thin Films with a Zero-Resistance Critical Temperature of 110K").
In the above method (1), c-axes of the Bi oxide superconductive material are perpendicularly oriented by heat treatment with respect to the base material. However, particles of the deposited film, which are grown along solid phase reaction, react at arbitrary growth rates in various sizes, whereby the film is significantly irregularized and deteriorated in smoothness and denseness. If an oxide superconductive material is thus deteriorated in denseness, it is impossible to improve critical current density.
In the method (2), on the other hand, improvement of critical current density is limited since grain growth occurs during conversion from the Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 phase to the Bi.sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 phase. A portion of unwanted film composition is caused by such change of phase such that a foreign phase is developed.
Further, in the method (2) metal lead is laid on the Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 phase. Such metal lead heterogeneously reacts when the same is heated to a temperature exceeding its melting point of 327.4.degree. C., so as to cause heterogeneous crystal grain growth. This also limits improvement of critical current density.
Thus, when a BiSrCaCuO superconducting film is prepared by a vapor phase method, it is difficult to obtain an oxide superconducting film having high critical current density of superconductivity which film is also excellent in crystal orientation and denseness.